Our discovery of cytonemes, specialized filopodia that orient toward cells that express signaling proteins, has led us to propose a novel mechanism that moves signaling proteins between producing and target cells. This model postulates that signaling proteins move between cells in a manner similar to the way neurotransmitters move at neuronal synapses - by transferring from pre- to post-synaptic cell at synapses. Work from my lab now shows that these non-neuronal synapses exist, that they involve proteins that have previously been shown to function and to be required at neuronal synapses, and are essential for paracrine signaling between non- neuronal cells. This direct and convincing experimental evidence firmly establishes that cytonemes ferry signaling proteins between producing and receiving cells. It also identified several unexpected properties of cytonemes that have significant implications for mechanisms of signal transduction. The work proposed in this application will develop new tools for imaging cytonemes and will build upon our previous findings to determine the roles, composition and functions of these remarkable organelles.